KR20140138235A - D2d connection recovery schemes - Google Patents

Abstract

Schemes for recovering a D2D connection between terminals (UEs) in an LTE network are described. The schemes can be applied to situations where a D2D connection is established between two UEs but for example one of the UEs is lost by entering the sleep state. These schemes can be applied to either centralized or distributed D2D systems. Some of the schemes are eNB-assisted and others are performed independently by the UEs.

Description

D2D connection recovery schemes {D2D CONNECTION RECOVERY SCHEMES}

[Related Applications]

This application is related to U.S. Provisional Patent Application No. 13 / 751,252, filed January 28, 2013, which claims the benefit of priority to U.S. Provisional Patent Application No. 61 / 624,185, filed April 13, The entire contents of which are incorporated herein by reference.

Device-to-device communication (D2D) is one means for improving the performance of LTE (Long Term Evolution) and other cellular networks. In D2D communication, terminals (referred to as user equipment or UEs in LTE) communicate directly with each other rather than through a base station (referred to as evolved node B or eNB in LTE). D2D communication between two or more D2D devices uses very low transmit power because it is typically local due to the short distance between the D2D devices. D2D communication is also a powerful way to increase the spatial reuse of resources of cellular systems for higher throughput. The present disclosure deals with aspects of D2D communication relating to recovery of lost connections between D2D devices.

1 shows an exemplary D2D communication UE devices and an eNB.
Figure 2 illustrates a scheme for D2D connection recovery in one embodiment.
Figure 3 illustrates a scheme for D2D connection recovery in one embodiment.
4 illustrates a scheme for D2D connection recovery in one embodiment.
5 illustrates a scheme for D2D connection recovery in one embodiment.
6 illustrates a scheme for D2D connection recovery in one embodiment.

The following description and drawings fully illustrate specific embodiments in order to enable those skilled in the art to practice the particular embodiments. Other embodiments may include structural, logical, electrical, process, and other modifications. Portions and features of certain embodiments may be included in other embodiments, or may be used instead. The embodiments described in the claims include all possible equivalents of the claims.

D2D communication as an underlay for an LTE network can be implemented as either a centralized or distributed system. In the latter case, UEs operating as D2D devices communicate directly without any eNB involvement. Such a distributed architecture is only relevant if it is out-of-band, where D2D communication uses resources other than the allowed spectrum. When a D2D communication occurs in the same authorized band as an LTE cellular communication, a centralized system is needed in which the eNB maintains control of radio resources. That is, the eNB is responsible for allowing D2D communication to occur between UEs and for allocating time-frequency resources to be used for D2D links.

In centralized or distributed D2D systems, there is an overhead involved in initially establishing a D2D link between two UE devices. If the established D2D connection between the two devices is lost, it would be desirable to have mechanisms to efficiently re-establish the D2D connection to avoid the overhead associated with establishing the connection. The schemes for D2D connection recovery for either centralized or distributed D2D systems are described herein. Some of the schemes are eNB-assisted, others are performed independently by the UEs.

System Description

Figure 1 shows an example of UE D1 and UE D2, each including a processor 21 interfaced with a radio-frequency (RF) transceiver circuit 22 connected to one or more antennas 23. The base station or eNB 40 is shown having a processor 41 interfaced with an RF transceiver circuit 42 connected to a plurality of antennas 43. The illustrated components are intended to provide air interfaces for both LTE and D2D communications as described above and to represent any type of hardware / software configuration for performing processing functions. In the illustrated embodiments, both UEs D1 and D2 communicate with eNB 40 over LTE links and communicate with each other over a D2D link.

D2D connection recovery schemes

In some embodiments, the D2D connection recovery scheme is suitable for situations where a UE with an established D2D connection goes into a sleep state (i.e., power saving mode). In this case, the D2D connection recovery can be implemented as a non-eNB assisted type or an eNB assisted type scheme.

In the example of a non-eNB assisted D2D connection recovery scheme, it is assumed that UEs D1 and D2 have an established D2D connection. D2 can notify UE D1 of a T _wakeup when it is no longer in sleep mode, corresponding to either the duration of the sleep state or the actual wake up time, when attempting to enter a power save mode have. Optionally, D2 may also be transmitted to the eNB using an uplink control channel. D1 may acknowledge that it has received a sleep message to inform D2 that it will be available when D2 wakes up. After receiving the acknowledgment message from D1, D2 may enter its sleep state, and entering the sleep state at this point may be subject to receiving an acknowledgment from D1. After a time of T _sleep , D2 wakes up and sends a D2D recovery request to D1. Then D1 responds with a D2D recovery response, and the D2D connection is restored. D2 may or may not notify the eNB of the D2D connection recovery.

Because of the control overhead associated with eNB operations, the less involvement of the eNB in the D2D connection recovery process, the better in terms of efficiency. However, in some cases, D2 and D1 can not negotiate before one of the devices enters the sleep state or otherwise enters the inactive state. For example, one device may experience abnormal power degradation. Alternatively, there may be an error in transmitting or receiving the sleep state notification, or in negotiating the agreed sleep time T _sleep . In these and other such cases, the eNB may be configured to help repair the D2D connection, as described below. It is assumed that the D2D connection between D1 and D2 is lost. D1 wants to restore the D2D connection to D2, but has no knowledge of when D2 will wake up. D1 sends a D2D recovery request to the eNB, and the eNB checks the status of D2. If D2 is in idle mode, the eNB will inform D1 of when to retry during the next paging cycle of D2. During the paging cycle, the eNB notifies D2 to enter an access mode for D2D recovery, and requests D1 to monitor and measure the uplink control channel of D2. Based on the measurement report sent to the eNB by D1, the eNB determines whether the D2D connection can be restored. If the answer is yes, the eNB notifies both D1 and D2 to proceed with a D2D connection recovery.

In another embodiment, when D1 wants to recover the D2D connection to D2, but there is no knowledge of when D2 wakes up, it sends a D2D recovery request to the eNB. If the eNB has received a T _wakeup notification from D2, the eNB will send a T _wakeup to D1. D1 will attempt to perform D2 and D2D reconnection at that time. If the eNB does not have such information about the T _wakeup , the eNB may notify D1 of the paging time of D2, and therefore D1 may attempt to reconnect to the D2D during the paging cycle of D2 as described above.

In another embodiment, when D1 sends a D2D restoration request to the eNB, if the eNB has received a T _wakeup notification from D2, the eNB may respond to D1 with both the paging time and the time T _wakeup of D2. D1 can choose between time T _wakeup and paging time as to when to try to recover the D2D connection. For example, if the paging time is much faster than T _wakeup , the former will be selected if the D2D reconnection is more urgent, for example, if less latency is required. D1 may be notified to the eNB on indicator parameter T _{reconnect _} D2D reconnection time selected with the _selected time or as to whether the paging T _wakeup is selected. If a paging cycle time is used for the D2D reconnect, the eNB may request D1 to perform a measurement of D2 for transmit power adjustment during the paging cycle. The eNB may then page to D2 as an indication of the D2D reconnection requested by D1. When D2 enters the connect mode, both D1 and D2 recover their D2D connection.

FIG. 2 illustrates an exemplary scheme for D2D connection recovery between UEs D1 and D2 that does not involve eNB 40. FIG. In step 201, a D2D connection is established between D1 and D2. In step 202, D2 notifies D1 that D2 is entering the sleep state and is awake at time T _wakeup . D2 may also optionally notify the eNB at step 204. [ At step 205, D1 acknowledges receipt of the message and informs D2 that it is awake at time T _wakeup . Step 206 indicates that both D1 and D2 are awake at time T _wakeup . In step 207, D1 sends a D2D connection recovery request to D2, which responds to the request in step 208. [ (Alternatively, D2 may send a D2D connection recovery request to D1.) In step 209, the D2D connection is restored.

FIG. 3 illustrates an exemplary scheme for D2D connection recovery between UEs D1 and D2 involving eNB 40. FIG. In step 301, a D2D connection is established between D1 and D2. At step 302, D2 notifies the eNB that D2 is entering the sleep state and is awake at time T _wakeup , but does not notify D1. In step 303, D1 notifies the eNB of a D2D recovery request that it wants to re-establish the D2 and D2D connection. In step 304, the eNB sends a _wakeup time T _wakeup of D2 to D1. Then D1 may or may not enter the sleep state, but both D1 and D2 are awake at step 305. < RTI ID = 0.0 > In step 306, D1 sends a D2D connection recovery request to D2, and in step 307, D2 responds to the request. (Alternatively, D2 may send a D2D connection recovery request to D1.) In step 308, the D2D connection is restored. In step 309, either D1 or D2 may selectively notify the eNB that the D2D connection has been restored.

4 illustrates an exemplary scheme for D2D connection recovery between UEs D1 and D2 involving eNB 40 when D2 enters its sleep state without notifying its wake-up time to either D1 or eNB. Lt; / RTI > In step 401, a D2D connection is established between D1 and D2, and subsequently D2 enters a sleep state without notifying D1 or eNB. At step 402, D1 sends a D2D recovery request to the eNB stating that it wants to re-establish the D2 and D2D connection. In response, the eNB at step 403 sends the next paging time for D2 to D1. At step 404, D1 can enter the sleep state, but is awake before the paging time for D2. At step 405, Dl sends a D2D recovery request back to the eNB stating that it wants to re-establish the D2 and D2D connection. At step 406, the eNB requests D1 to monitor and measure the transmission of D2 when responding to its paging through the physical random access channel (PRACH) or transmitting on the uplink control channel. At step 407, D2 is paged by the eNB. D2 performs the network re-entry procedure via the PRACH in step 408. [ At step 409, Dl sends its measurement report to D2 transmissions to the eNB. Based on the measurement report at step 410, if the eNB determines that the channel quality between D1 and D2 is adequate to support the D2D link, a D2D recovery permission is sent to both D1 and D2. In step 411, D1 sends a D2D connection recovery request to D2, and in step 412, D2 responds to the request. (Alternatively, D2 may send a D2D connection recovery request to D1.) In step 413, the D2D connection is restored.

5 illustrates an exemplary scheme for D2D connection recovery between UEs D1 and D2 involving eNB 40 when D2 enters its sleep state after informing its eNB of its wake-up time. In step 501, a D2D connection is established between D1 and D2, and subsequently D2 notifies the eNB of its wake-up time T _wakeup in step 502 but goes into a sleep state without notifying D1. At step 503, D1 sends a D2D recovery request to the eNB stating that it wants to re-establish the D2 and D2D connection. In response, at step 504, the eNB sends D1 the next paging time for D2 and the wakeup time T _wakeup of D2. At step 505, D1 selects the paging time of D2 (assuming it is earlier than the time T _wakeup ), notifies the eNB and enters the sleep state until just before the paging time of D2. At step 506, D1 again sends a D2D recovery request to the eNB stating that it wants to re-establish the D2 and D2D connection. In step 507, the eNB requests D1 to monitor and measure the transmission of D2 when responding to its paging through the physical random access channel (PRACH) or transmitting on the uplink control channel. At step 508, D2 is paged by the eNB. At step 509 D2 performs a network re-entry procedure over the PRACH and at step 510 D1 sends its measurement report for D2's transmissions to the eNB. Based on the measurement report, if the eNB determines that the channel quality between D1 and D2 is adequate to support the D2D link, then in step 511 a D2D recovery permission is sent to both D1 and D2. At step 512, D1 sends a D2D connection recovery request to D2, and at step 513 D2 responds to the request. (Alternatively, D2 may send a D2D connection recovery request to D1.) At step 514, the D2D connection is restored.

Figure 6 shows an exemplary scheme for D2D connection recovery between UEs D1 and D2 involving eNB 40 similar to the scheme shown in Figure 5, but in this case, to reestablish the D2 and D2D connections, D1 chooses its wake-up time rather than D2's paging time, because it is earlier, or for some other reason. In step 601, a D2D connection is established between D1 and D2, and subsequently D2 notifies the eNB of its wake-up time T _wakeup in step 602 but goes into a sleep state without notifying D1. At step 603, D1 sends a D2D recovery request to the eNB stating that it wants to re-establish the D2 and D2D connection. In response, in step 604, the eNB sends D1 the next paging time for D2 and the wake-up time T _wakeup of D2. At step 605, D1 selects the time T _wakeup (assuming it is earlier than the paging time of D2) and notifies the eNB and enters the sleep state until just before time T _wakeup . In step 607, D1 sends a D2D connection recovery request to D2, and in step 608 D2 responds to the request. (Alternatively, D2 may send a D2D connection recovery request to D1.) In step 609, the D2D connection is restored.

Exemplary embodiments

In one embodiment, an apparatus operating as a user equipment (UE) in an LTE network comprises a wireless transceiver for communicating with the eNB and providing an air interface for D2D communication, and a processing coupled to the wireless transceiver Wherein the processing circuit establishes a D2D communication session with the second UE and if a message is received from the second UE indicating that the second UE will enter the sleep state and indicate the wake up time, Message to the second UE, sends the D2D recovery message to the second UE at the indicated wake-up time, and waits for a response to it to reestablish the D2D communication session. The processing circuit also sends a D2D recovery request to the eNB if a second UE enters a sleep state but no message indicating a wake up time is received from the second UE; If a response to the D2D restoration request indicating the next paging time is received from the eNB and the second UE responds to the page at the paging time, then a D2D recovery message is sent to the second UE at the paging time, and the D2D communication session You can wait for a response to re-establish it. The processing circuit also sends a D2D recovery request to the eNB if a second UE enters a sleep state but no message indicating a wake up time is received from the second UE; If a response to the D2D restoration request indicating a wake up time previously sent to the eNB by the second UE is received from the eNB, it sends a D2D recovery message to the second UE at wake up time and re-establishes the D2D communication session You can wait for a response to it. The processing circuit may also be configured to: when requested by the eNB, measure a signal sent by the second UE when responding to the page over a physical random access channel (PRACH), and send a measurement report to the eNB; It may send a D2D recovery message to the second UE at the paging time and wait for a response to it to reestablish the D2D communication session only if it is authorized by the eNB in response to the transmitted measurement report. The processing circuit may also be configured to: when requested by the eNB, measure a signal transmitted by the second UE on an uplink control channel after a response to the page, and transmit a measurement report to the eNB; If granted by the eNB in response to the transmitted measurement report, it may send a D2D recovery message to the second UE at the paging time and wait for a response to it to reestablish the D2D communication session.

In another embodiment, an apparatus operating as a user equipment (UE) in an LTE network includes a wireless transceiver for communicating with the eNB and providing an air interface for D2D communication, and a processing circuit coupled to the wireless transceiver , The processing circuit establishing a D2D communication session with the first UE; If it enters a sleep state after establishing a D2D communication session, sending a message indicating a wake up time to the first UE; If an acknowledgment message is received from the first UE in response to the transmission of the wake-up time, and a D2D recovery request is received from the first UE at the wake-up time, the D2D recovery request is re- Reply. The processing circuitry may also send a message to the eNB indicating a wake up time if it enters a sleep state after establishment of the D2D communication session. The processing circuitry may also send a message to the eNB indicating that the D2D communication session has been re-established after re-establishment of the D2D communication session with the first UE.

In another embodiment, a method for operating an eNB in an LTE network includes receiving a D2D message from a first UE and a second UE in response to channel reservation requests received from a first user equipment (UE) and a second user equipment Allocating time-frequency resources for communication; And a D2D communication session between the first UE and the second UE is established and after the second UE enters the sleep state, by sending a paging time for the second UE to the first UE, And responding. The method also includes setting a wake-up time for the second UE to a first time after a D2D communication session between the first UE and the second UE is established, the second UE enters a sleep state and transmits a wake-up time to the eNB, And responding to the D2D recovery request from the first UE by sending to the UE. The method may further comprise sending a page to the second UE at a paging time with an indication of a D2D recovery request from the first UE after sending the paging time for the second UE to the first UE have. The method also includes measuring a first request to the first UE to measure a signal transmitted by the second UE when responding to the page via a physical random access channel (PRACH) after receiving a D2D recovery request from the first UE, And sending the request. The method further includes the step of requesting the first UE to measure the signal transmitted by the second UE after receiving the D2D recovery request from the first UE and then responding to the page over the uplink control channel can do. The method also includes receiving a measurement report from the first UE in response to the measurement request, then determining whether the measured signal is appropriate, and transmitting the D2D recovery permission to the first UE and the second UE .

In another embodiment, a method of operating a UE in an LTE network includes establishing a D2D communication session with a second UE; Negotiating a wake-up time at which the second UE will wake up from the sleep state if a message is received from the second UE indicating that the second UE will enter the sleep state; And sending the D2D recovery message to the second UE and waiting for a response thereto to re-establish the D2D communication session. The method also includes sending a D2D recovery request to the eNB if the second UE enters a sleep state but no message indicating a wake up time is received from the second UE; And then a D2D recovery message indicating the paging time is received from the eNB, and if the second UE responds to the page at the paging time, the D2D recovery message is sent to the second UE at the paging time, and the D2D communication And waiting for a response to re-establish the session. The method also includes sending a D2D recovery request to the eNB if the second UE enters a sleep state but no message indicating a wake up time is received from the second UE; And if a response to the D2D restoration request indicating a wake up time previously sent to the eNB by the second UE is received from the eNB, send a D2D recovery message to the second UE at wake up time, And waiting for a response to establish it. The method also includes the steps of: when requested by the eNB, measuring a signal transmitted by the second UE when responding to the page via a physical random access channel (PRACH) and sending a measurement report to the eNB; And sending a D2D recovery message to the second UE at the paging time only if it is authorized by the eNB in response to the transmitted measurement report and waiting for a response thereto to re-establish the D2D communication session . The method also includes the steps of: when requested by the eNB, measuring a signal transmitted by the second UE over the uplink control channel after a response to the page and sending a measurement report to the eNB; And sending a D2D recovery message to the second UE at the paging time and waiting for a response to it to reestablish the D2D communication session if it is granted by the eNB in response to the transmitted measurement report. The method also includes establishing a D2D communication session with the first UE; Transmitting a message indicating a wake up time to a first UE if the UE enters a sleep state after establishing a D2D communication session; And if an acknowledgment message is received from the first UE in response to the transmission of the wake up time and a D2D recovery request is received from the first UE at the wake up time, then a D2D recovery request is sent to re-establish the D2D communication session And a step of responding to the request.

The embodiments described above may be implemented with various hardware configurations that may include a processor for executing instructions as methods for operation and / or methods. Such instructions may be included in a suitable storage medium, and instructions from the storage medium may be transferred to memory or other processor-executable media.

This point has been described in the context of LTE networks. Except in case of inconsistency, the gist can be used for other types of cellular networks by replacing references to UEs and eNBs with references to terminals and base stations, respectively.

The subject matter has been described in connection with the specific embodiments above. It will be appreciated that these embodiments may be combined in any way that is also considered to be advantageous. In addition, many alternatives, modifications, and variations will be apparent to those skilled in the art. Other such alternatives, modifications, and variations are intended to fall within the scope of the appended claims.

37 C.F.R. requiring an abstract that allows the reader to identify the nature and substance of the disclosure. A summary is provided to comply with Section 1.72 (b). The summary is presented in the understanding that it will not be used to interpret or limit the scope or meaning of the claims. The following claims are each included as a separate embodiment in the detailed description herein independently.

Claims (20)

1. A user equipment (UE) device,
a wireless transceiver for communicating with an evolved node B (eNB) and providing an air interface for device-to-device (D2D) communication; And
And a processing circuit connected to the wireless transceiver,
Establish a D2D communication session with the second UE,
Transmitting from the second UE an acknowledgment message to the second UE when the second UE enters a sleep state and a message indicating a wake up time is received, Sends a D2D recovery message to the second UE and waits for a response thereto to re-establish the D2D communication session. The method according to claim 1,
The processing circuit may further comprise:
Send a D2D recovery request to the eNB if the second UE enters a sleep state but a message indicating the wake up time is not received from the second UE;
If a response to the D2D recovery request indicating a next paging time is received from the eNB and if the second UE responds to the page at the paging time, And waits for a response thereto to re-establish the D2D communication session. The method according to claim 1,
The processing circuit may further comprise:
Send a D2D recovery request to the eNB if the second UE enters a sleep state but a message indicating the wake up time is not received from the second UE;
Send a D2D recovery message to the second UE at the wakeup time if a response to the D2D recovery request indicating a wake up time previously sent to the eNB by the second UE is received from the eNB, Waiting for a response to the D2D communication session to reestablish the D2D communication session. 3. The method of claim 2,
The processing circuit may further comprise:
Measure a signal sent by the second UE when responding to the page via a physical random access channel (PRACH), upon request by the eNB, and send a measurement report to the eNB;
Sending a D2D recovery message to the second UE at the paging time only if it is authorized by the eNB in response to the transmitted measurement report and waiting for a response to re-establish the D2D communication session; (UE) device. 3. The method of claim 2,
The processing circuit may further comprise:
If requested by the eNB, measure a signal transmitted by the second UE on an uplink control channel after a response to the page, and send a measurement report to the eNB;
(UE), which, when granted by the eNB in response to the transmitted measurement report, sends a D2D recovery message to the second UE at the paging time and waits for a response thereto to re-establish the D2D communication session. ) Device. 1. A user equipment (UE) device,
a wireless transceiver for communicating with an evolved node B (eNB) and providing an air interface for device-to-device (D2D) communication; And
And a processing circuit connected to the wireless transceiver,
Establish a D2D communication session with the first UE;
Send a message indicating the wake up time to the first UE if it enters a sleep state after establishing the D2D communication session;
If an acknowledgment message is received from the first UE in response to the transmission of the wake up time and a D2D recovery request is received from the first UE at the wake up time, And responds to the D2D recovery request. The method according to claim 6,
The processing circuit may further comprise:
And transmits a message indicating the wake up time to the eNB if it enters a sleep state after establishing the D2D communication session. The method according to claim 6,
The processing circuit may further comprise:
The UE sends a message to the eNB indicating that the D2D communication session has been re-established after re-establishment of the D2D communication session with the first UE. A method for operating an evolved node B (eNB) in an LTE (Long Term Evolution) network,
In response to D2D channel reservation requests received from a first user equipment (UE) and a second user equipment (UE), a time-frequency (D2D) message for device-to-device communication between the first UE and the second UE Allocating resources; And
Wherein a D2D communication session between the first UE and the second UE is established and after the second UE enters a sleep state, by sending a paging time for the second UE to the first UE, And responding to the D2D recovery request of the eNB. 10. The method of claim 9,
After a D2D communication session between the first UE and the second UE is established and the second UE enters a sleep state and transmits a wake up time to the eNB, Lt; RTI ID = 0.0 > 1 < / RTI > UE by responding to the D2D recovery request from the first UE. 10. The method of claim 9,
Further comprising: after sending the paging time for the second UE to the first UE, sending a page to the second UE at the paging time with an indication of the D2D recovery request from the first UE , eNB operating method. 10. The method of claim 9,
Requesting the first UE to measure a signal transmitted by the second UE when responding to the page via a physical random access channel (PRACH) after receiving a D2D recovery request from the first UE, Further comprising the step of: 10. The method of claim 9,
Further comprising: after receiving the D2D recovery request from the first UE, requesting the first UE to measure the signal transmitted by the second UE after responding to the page on the uplink control channel , eNB operating method. 13. The method of claim 12,
After receiving a measurement report from the first UE in response to the measurement request, determining whether the measured signal is appropriate, and transmitting a D2D recovery permission to the first UE and the second UE, Further included is an eNB operating method. A method of operating a user equipment (UE)
Establishing a device-to-device (D2D) communication session with the second UE;
Negotiating, from the second UE, a wake-up time at which the second UE wakes from the sleep state when a message indicating that the second UE enters a sleep state is received; And
Sending a D2D recovery message to the second UE and waiting for a response thereto to re-establish the D2D communication session. 16. The method of claim 15,
Sending a D2D recovery request to the eNB if the second UE enters a sleep state but a message indicating the wake up time is not received from the second UE; And
If a response to the D2D recovery request indicating a next paging time is received from the eNB and if the second UE responds to the page at the paging time, And waiting for a response to the D2D communication session to reestablish the D2D communication session. 16. The method of claim 15,
Sending a D2D recovery request to the eNB if the second UE enters a sleep state but a message indicating the wake up time is not received from the second UE; And
Send a D2D recovery message to the second UE at the wakeup time if a response to the D2D recovery request indicating a wake up time previously sent to the eNB by the second UE is received from the eNB, Further comprising waiting for a response thereto to re-establish the D2D communication session. 17. The method of claim 16,
Measuring a signal transmitted by the second UE when responding to the page via a physical random access channel (PRACH) when requested by the eNB, and transmitting a measurement report to the eNB; And
Sending a D2D recovery message to the second UE at the paging time only if it is granted by the eNB in response to the transmitted measurement report and waiting for a response to it to reestablish the D2D communication session (UE). ≪ / RTI > 17. The method of claim 16,
When requested by the eNB, measuring a signal transmitted by the second UE on an uplink control channel after a response to the page, and transmitting a measurement report to the eNB; And
Sending a D2D recovery message to the second UE at the paging time and waiting for a response to it to reestablish the D2D communication session if it is granted by the eNB in response to the transmitted measurement report , A method of operating a user equipment (UE). 16. The method of claim 15,
Establishing a D2D communication session with the first UE;
Sending a message to the first UE indicating a wake-up time if it enters a sleep state after establishing the D2D communication session; And
If an acknowledgment message is received from the first UE in response to the transmission of the wake up time and a D2D recovery request is received from the first UE at the wake up time, And responding to the D2D recovery request.

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